It is very well known in the art to provide devices of the above type. The desired effect of such devices is to shift weight to the front axle of the tractor away from the rear axle of the trailer, in order to meet legal load limitations on the various axles and to improve handling of the tractor. The devices in question are embodied by pneumatic means exerting upward force on the forward end of the trailer. Till now, such devices have been located forwardly of the fifth wheel of the tractor.
However, it is not always advantageous to shift weight away from the driving wheels of the tractor. For example, a tractor trailer unit travelling on a rough dirt or sand forest road with insufficient weight on the driving wheels of the tractor may undergo "wheel jumping" and/or differential breakdown. Poor traction is also produced on snow- or ice-covered roads, or on forest roads. Poor traction necessitates the use of deeply-grooved tires which are expensive, or chains which are time-consuming to install and remove.